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How to Calculate Water Volume

By Teresa J. Siskin; Updated September 29, 2017

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Calculating water volume depends on the shape of the vessel containing the water. Volume calculations for a square or rectangular vessel require you know its length, width and depth for entry into the volume equation, V=L_W_D, where L stands for length, W is the width and D represents depth. Performing the same volume calculation for a circular vessel requires you to know the vessel’s depth and diameter or radius, as the volume equation in this instance reads: V= (r^2)Dπ, where r represents the radius, D stands for depth, and π, or pi, equals a constant value commonly rounded to 3.14.

Rectangular Vessel Volume

Given a swimming pool that measures 10 feet long, 12 feet wide and a uniform 6 feet deep, calculate the volume of water in that pool my multiplying 10 by 12 by 6, which yields 720 cubic feet of water. One cubic foot is equal to 7.48 gallons, meaning the volume of water in this swimming pool would equal approximately 5,386 gallons of water.

Circular Vessel Volume

If you must calculate the volume of water in a round pool that is 20 feet in diameter and 6 feet deep, begin my halving the diameter to achieve the radius length, which is 10. Square the radius and multiply it by the depth and by the constant pi, or 3.14. This yields a volume of 1,884 cubic feet of water, which, when multiplied by 7.48 to convert cubic feet to gallons, equals roughly 14,092 gallons of water.